QUESTIONI have 2x2 inch mosaic tile in a handicap shower I am having trouble with water seeping through the tile grout an leaking in the crawl space below. What would be the best way to seal the grout? Thank you for your answer
ANSWERANSWER - If water from your shower is leaking into a crawl space the problem is not the cementitious grout joints. There has to be an underlying problem with the shower pan's waterproof membrane. It likely was not installed correctly.
To waterproof a shower pan and shower walls, you have to have a waterproof membrane that meets ANSI A118.10 to be properly installed. The plumbing code says that the waterproof shower pan liner has to extend up at least 3 inches above the height of the top surface of the dam/curb. Along with having a moisture barrier sheeting behind the wall substrate so it weather laps over the shower pan liner.
Apparently your waterproof membrane is not working as it should or you wouldn't have a leak into the crawl space. The proper way to fix this problem is to remove at least the shower pan and reinstall it correctly.
If you have a limited budget and want to take a risk, there is a possible fix that doesn't meet the standards. You could remove the cementitious grout from the grout joints, remove the grout in the shower pan floor to wall horizontal transitions, and remove the grout in the inside corners of the wall tile.
The once the joints are open and clean, use an ASTM C920 silione sealant and fill all of those joints. If your 2x2 inch tile is porcelain and you have these grout joints filled with sealant, in theory, if you do everything correctly, water should not be able to pass through. You might have to do the same thing with all of the grout joints on the wall tile.
This method doesn't come with a guarantee and only proceed at your own risk.